One of the world's greatest traders once said "There will always be trends in the markets as long as there are people who can't read them". That certainly doesn't answer your question, but your thread just reminded me of this saying which I like. You'll probably get a variety of answers. But if someday you learn to identify the beginning and end of trends, remember to trade the middle. Just as with roast beef, that's where the best part is!!
if we talking about uptrend it starts at support and ends at resistance, opposite is true for the downtrend